A conventional dockboard, which is employed to bridge the gap between a loading dock and the bed of a truck or carrier, includes a ramp having its rear end hinged to the dock. The ramp is biased upwardly to an inclined position by a counterbalancing mechanism and a manually releasable holddown device holds the ramp against elevation. Hinged to the front edge of the ramp is an extension lip which is adapted to swing from a pendant position, in which the lip hangs downwardly, to an elevated position, in which the lip forms an extension to the ramp. The lip, when in the extended position, is adapted to engage the bed of a truck to bridge the gap between the ramp and the truck to enable material handling equipment to move between the dock and the truck bed.
U.S. Pat. No. 4,922,568 describes a hydraulic holddown mechanism that includes a cylinder connected to the ramp, and a piston is slidable in the cylinder and carries a piston rod that is connected to the frame of the dockboard. The piston is provided with a longitudinal passage which is closed off by a valve member and an actuating rod connected to valve member extends outwardly of the cylinder. Movement of the actuating rod will open the valve to permit extension of the piston rod with respect to the cylinder to enable the ramp to be pivoted upwardly by the counterbalancing spring assembly.
If the ramp is at a below-dock-level position when the truck pulls away from the dock, the ramp will remain at this downwardly inclined position unless the holddown mechanism is manually released to enable the counterbalancing spring assembly to raise the ramp. With the ramp elevated, the operator will then walk outwardly on the ramp, and the weight of the operator in combination with the weight of the ramp will lower the ramp until it returns to its horizontal cross traffic position.
It has been found that occasionally the operator may not carry out this procedure to return the ramp to dock-level position, with the result that the ramp remains in the downwardly inclined position. With the ramp in this below-dock-level position, it can provide an obstruction to material handling equipment moving over the dock.
U.S. Pat. Nos. 3,699,601 and 3,967,337 describe mechanisms which will automatically return the ramp to a dock-level position if the ramp is at a below-dock-level position when the truck pulls away from the dock. In these patents, a mechanism is employed which acts to release the holddown mechanism as the lip pivots downwardly from the extended position toward the pendant position. Release of the holddown mechanism enables the counterbalancing spring assembly to pivot the ramp upwardly to the dock-level position.